The invention relates to a process for the catalytic hydrodesulfurization of heavy hydrocarbon oils.
Heavy hydrocarbon oils such as residues obtained in the distillation of crude petroleum at atmospheric or reduced pressure generally contain a considerable quantity of sulfur compounds. In order to reduce the sulfur content of the heavy oils they may be subjected to a catalytic hydrodesulfurization treatment. This treatment typically is carried out by contacting the heavy oil, together with hydrogen, at elevated temperature and pressure with a catalyst which contains one or more metals having hydrogenative activity, supported on a carrier. One drawback to this direct desulfurization route is that a fairly rapid deactivation of the catalyst generally occurs. This catalyst deactivation is caused, inter alia, because the above-mentioned heavy hydrocarbon oils generally contain a considerable quantity of vanadium compounds, which are deposited on the catalyst during the desulfurization process. As the catalyst activity declines, a higher temperature has to be used in order to maintain the desired degree of desulfurization. In practice, the procedure generally followed is to initiate the process at the lowest possible temperature at which the desired degree of desulfurization is just attained.
In an investigation carried out by the Applicant into the catalytic hydrodesulfurization of vanadium-containing heavy hydrocarbon oils it has been found that the catalyst deactivation which occurs as a result of the deposition of vanadium on the catalyst can be partly compensated by carrying out the process in the presence of a quantity of water vapor corresponding with a water vapor partial pressure during the process of 0.5-30 bar. In addition to the above-mentioned favorable effect on catalyst activity of the presence of water vapor, there are also two less attractive aspects attached to carrying out the catalytic hydrodesulfurization of the present vanadium-containing heavy oils in the presence of water vapor. In the first place, the use of water vapor requires extra energy in order to evaporate the requisite quantity of water, resulting in a rise in the costs of the desulfurization process. Further, in order to enable the process to be carried out at a constant total pressure, the hydrogen partial pressure must be reduced if the desulfurization is carried out in the presence of water vapor. However, reduction of the hydrogen partial pressure during the catalytic hydrodesulfurization of the present heavy oils generally entails lower catalyst activity.
Continued investigation into this subject revealed that in the initial phase of the process, when only a small quantity of vanadium has been deposited on the catalyst and the catalyst deactivation caused by vanadium deposition is therefore still slight, the favorable effect of water vapor on catalyst activity can easily be offset by the two abovementioned less attractive aspects of the use of water vapor. Applicant has found that based upon economic considerations the use of water vapor in the catalytic hydrodesulfurization of vanadium-containing heavy hydrocarbon oils only begins to become attractive once the average vanadium content of the catalyst has increased by at least 5 parts by weight in a preceding operation without the use of water vapor (vanadium content of the catalyst expressed in parts by weight of vanadium per 100 parts by weight of carrier material).